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This is the peer reviewed version of the following article: Mothe, S. R.; Tan, J. S. J.; Chennamaneni, L. R.; Aidil, F.; Su, Y.; Kang, H. C.; Lim, F. C. H.; Thoniyot, P., A systematic investigation of the ring size effects on the free radical ring-opening polymerization (rROP) of cyclic ketene acetal (CKA) using both experimental and theoretical approach. Journal of Polymer Science DOI: 10.1002/pol.20200210., which has been published in final form at https://doi.org/10.1002/pol.20200210. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.

Room temperature growth of HfO2 thin film on clean 2H-MoS2 via plasma-sputtering of Hf-metal target in an argon/oxygen environment was studied in-situ using x-ray photoelectron spectroscopy (XPS). The deposited film was observed to grow akin to a layer-by-layer growth mode. At the onset of growth, a mixture of sulfate- and sulfite-like species (SOx2− where x = 3, 4), and molybdenum trioxide (MoO3), are formed at the HfO2/MoS2 interface. An initial decrease in binding energies for both Mo 3d and S 2p core-levels of the MoS2 substrate by 0.4 eV was also observed. Their binding energies, however, did not change further with increasing HfO2 thickness. There was no observable change in the Hf4f core-level binding energy throughout the deposition process. With increasing HfO2 deposition, MoO3 becomes buried at the interface while SOx2− was observed to be present in the film. The shift of 0.4 eV for both Mo 3d and S 2p core-levels of the MoS2 substrate can be attributed to a charge transfer from the substrate to the MoO3/SOx2−-like interface layer. Consequently, the Type I heterojunction valence band offset (conduction band offset) becomes 1.7 eV (2.9 eV) instead of 1.3 eV (3.3 eV) expected from considering the bulk HfO2 and MoS2 valence band offset (conduction band offset). The formation of these states and its influence on band offsets will need to be considered in their device applications.

In this work, real-time morphological evolution occurring on the GeSn(111) surface during in-situ annealing in a N 2 environment is examined using a phase contrast optical microscope equipped with a heating stage. Apart from isolated immobile Sn-rich dot-like islands, mobile teardrop-shaped islands are also found at the moving boundary separating the smooth and roughen regions on the surface. As the boundary grows, Sn-poor wire-like feature emerges. The rate of growth occurs with an activation energy of ~1.4eV and appears to be akin to liquid phase epitaxy where the Sn-rich droplets leads the formation of wire-like features and the movement is due to the directional flow of Ge atoms through the droplet. DFT calculations suggest that the morphological instability is thermodynamically driven, where the relaxation of the inherent strain occurs with Sn atoms preferentially segregating from the bulk-lattice and agglomerating at the surface. The formation of these heterogeneous surfaces would restrict the thermal budget for GeSn alloys to be monolithically integrated into Silicon or Germanium devices.

When graphene supported on SiO 2 is fluorinated, XPS reveals an increase in concentration of chemically-adsorbed fluorine (higher F/C ratio with C-CF, C-CF 2 , C-F and C-F 2 but no C-F 3 ) on the graphene surface with time. Raman I D /I G ratio, i.e. a measure of non-sp2to Sp2 bonding states, increases with time before showing a decrease suggesting a surface morphology change owing to C-F bonding followed by disordering of the π-electron system. AFM surface morphology scans reveal that defects (holes), which increases in size with time, are observed to form preferentially at the boundary of the graphene flakes. Synchronized Kelvin-Probe Force-Microscopy (KPFM) mapping of the graphene region surrounding these holes shows a higher work-function, ϕ, giving rise to a donut-shape contact potential difference (CPD) which increases from 4.9 ± 0.1 eV to 5.4 ± 0.1 eV with fluorination. Together with XPS and Raman results, the increase in ϕ can be attributed to the presence of a higher concentration of fluorine in the graphene region (C-F/C-F 2 bonds) surrounding these holes. The formation of the hole-defects on graphene and its subsequent increase in size with fluorination is thus a result of aggregation of adsorbed fluorine and removal of carbon likely in the form of CF 4 or C 2 F 4 .

The physical and chemical properties of transition metal nanoclusters have been extensively investigated. In particular, we study the energetics of the mixed clusters Pt4−nNin, focusing on the binding energy of the clusters Ebind to a graphene support, and the hydrogenation energy Eads in both the gas-phase and the graphene-supported clusters. For each cluster composition, the cluster can bind to graphene in either a face-on or an edge-on configuration, and in each of these orientations, binding can occur through different atoms; we explore these binding configurations comprehensively. We discuss the variation of Ebind and Eads with respect to the composition of the cluster and the binding configuration of the cluster to the graphene support. Our results show that hydrogen is generally chemisorbed at a Pt site and physisorbed at a Ni site, with a dependence of the adsorption energy upon the composition and the adsorption configuration. Compared with the gas-phase cluster, the chemisorption energies are ge...

Growth through controlled adsorption of ferromagnetic elements such as Fe, Co and Ni on two-dimensional silicene provides an alternative route for silicon-based spintronics. Plane wave DFT calculations show that Fe, Co and Ni adatoms are strongly chemisorbed via strong sigma bonds, with adsorption energies (1.55 - 2.29 eV) that are two to six times greater compared to adsorption on graphene. All adatoms adsorb more strongly at the hole site than at the atom site, with Ni adsorbing strongest. Of the dimer configurations investigated, the hole – hole, b-atom – hole, vertically stacked at hole, vertically stacked at b-atom and bridge sites were found to be stable. The Co and Ni dimers are most stable when adsorbed in the hole-hole configuration while the Fe dimer is most stable when adsorbed in the atom-hole configuration. Metal-to-silicene and interconfigurational s-to-d electron transfer processes underpin the trends observed in adsorption energies and magnetic moments for both adatoms and dimers. Adsorption of these metals induces a small band gap at the Dirac Cone. In particular Co adatom adsorption at the hole site induces the largest spin-polarized band gaps of 0.70 eV (spin-up) and 0.28 eV (spin-down) making it a potential material candidate for spintronics applications.

The behavior of water confined at the nanoscale between graphene sheets has attracted much theoretical and experimental attention recently. However, the interactions, structure, and energy of water at the molecular scale underpinning the behavior of confined water have not been characterized by first-principles calculations. In this work we consider small water clusters up to the hexamer adsorbed between graphene sheets using density functional theory calculations with van der Waals corrections. We investigate the effects on structure, energy, and intermolecular interactions due to confinement between graphene sheets. For interlayer distances of about one nanometer or more, the cluster adsorption energy increases approximately linearly with the cluster size by 0.1 eV per molecule in the cluster. As the interlayer distance decreases, the cluster adsorption energy reaches a maximum at 6 to 7 A with approximately 0.16 eV stabilization energy relative to large interlayer distances. This suggests the possibility of controlling the amount of adsorption in graphene nanomaterials by varying the interlayer distance. We also quantify the intermolecular hydrogen bonding in the clusters by calculating the dissociation energy required to remove one molecule from each cluster. For each cluster size, this is constant for interlayer distances larger than approximately 6 to 8 A. For smaller distances the intermolecular interaction decreases rapidly thus leading to weaker cohesion between molecules in a squeezed cluster. We expect a mechanism of concerted motion for hydrogen-bonded water molecules confined between graphene sheets, as has been observed for water confined within the carbon nanotubes. Thus, the decrease in the dissociation energy we observed here is consistent with experimental results for water transport through graphene and related membranes that are of interest in nanofiltration. We also calculate the corrugation in the interaction potential between graphene sheets which suggests a switch from very small corrugation to stick–slip behavior at interlayer distances smaller than 6 A. Our results for gas phase clusters agree reasonably with methods using more demanding quantum chemical methods to treat the van der Waals interactions, thus providing support for the relatively fast density functional theory methods used here for studying water–graphene interactions in nanoscale systems.

Recent work has shown that the most stable graphene-adsorbed dimers of Fe, Co, and Ni have top-atom magnetic moments larger than those of the free dimers. Here we investigate the dependence of the binding energy and the top-atom magnetic moment upon composition, cluster size, and cluster dimensionality by considering the trimer and the tetramer, that is, the smallest two-dimensional and three-dimensional clusters, respectively. We find that for trimers the highest binding energy occurs for configurations that are perpendicularly bound and have the largest charge transfer to graphene. For tetramers, the binding energy is highest for the compact configurations with the largest charge transfer to graphene. Binding is generally strongest at the hole site of the graphene lattice. The charge transfer to graphene is mainly from the base atoms, while the $s$-$d$ orbital configuration of the top atom is close to that in the free cluster, indicating electronic shielding of the top atom from the graphene substrate. Thus, the binding energy of mixed trimers and tetramers to graphene is determined largely by the elemental identity of the base atoms, while the magnetic moment of the top atom depends on the elemental identity of the top atom. We show that graphene-adsorbed mixed clusters FeCo${}_{2}$ and FeCo${}_{3}$ with top Fe atom are strongly bound and have large top-atom magnetic moments, indicating the potential for magnetic storage applications.

Car-Parrinello molecular dynamics (CPMD) are widely used to investigate the dynamical properties of molecular systems. An important issue in such applications is the dependence of dynamical quantities such as molecular vibrational frequencies upon the fictitious orbital mass μ. Although it is known that the correct Born-Oppenheimer dynamics are recovered at zero μ, it is not clear how these dynamical quantities are to be rigorously extracted from CPMD calculations. Our work addresses this issue for vibrational frequencies. We show that when the system is sufficiently close to the ground state the calculated ionic vibrational frequencies are ω(M) = ω(0M)[1 -C(μ/M)] for small μ/M, where ω(0M) is the Born-Oppenheimer ionic frequency, M the ionic mass, and C a constant that depends upon the ion-orbital coupling force constants. Our analysis also provides a quantitative understanding of the orbital oscillation amplitudes, leading to a relationship between the adiabaticity of a system and the ion-orbital coupling constants. In particular, we show that there is a significant systematic dependence of calculated vibrational frequencies upon how close the CPMD trajectory is to the Born-Oppenheimer surface. We verify our analytical results with numerical simulations for N(2), Sn(2), and H/Si(100)-(2×1).

We demonstrate the preferential formation and self-assembly of monodisperse Si magic clusters $({X}_{4})$ of size $\ensuremath{\sim}13.5\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{\AA{}}$ on $\text{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ surface using scanning tunneling microscope. The growth process is observed to occur via a stepwise assembly of planarized Si tetramers $({X}_{1})$ formed from Si adatoms deposited at room temperature, leading to Si tetraclusters $({X}_{2})$ (size $\ensuremath{\sim}4.6\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{\AA{}}$) and culminating in tetracluster dimer $({X}_{3})$ and trimer $({X}_{4})$ formations as the surface is being annealed progressively to $150\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. The respective cluster species density distribution at each annealing temperature also shows the preferential formation of ${X}_{1}\ensuremath{\rightarrow}{X}_{2}\ensuremath{\rightarrow}{X}_{3}\ensuremath{\rightarrow}{X}_{4}$ at higher temperatures, which we describe using surface reaction schemes; ${X}_{1}\ensuremath{\rightarrow}{X}_{2}$, ${X}_{2}+{X}_{2}\ensuremath{\rightarrow}{X}_{3}$, and ${X}_{2}+{X}_{3}\ensuremath{\rightarrow}{X}_{4}$. We determine the activation and formation energies for respective cluster species and elucidate the formation energetics and dynamics of tetraclusters which function unequivocally as fundamental building blocks in the self-assembly of stable Si magic clusters. Finally, we resolve the structure of the Si magic cluster to comprise three tetraclusters or $n=12$ Si atoms taking into consideration (i) cluster symmetry and alignment, (ii) close packing, and (iii) minimization of dangling bonds.

Metal clusters have been investigated rather intensely for both fundamental and technological reasons. In this work we report the results of plane-wave density functional theory calculations of Fe, Co, and Ni adatoms and dimers adsorbed on graphene. We study both homonuclear and heteronuclear dimers, and the latter includes mixed dimers of Fe, Co, and Ni along with dimers of these elements with Pt. Our work is motivated by the fundamental interest in their configurational and magnetic properties. We calculated the adsorption site, the structure and relative stabilities of various adsorption configurations, the band structures, the atomic projected electronic density of states, and the magnetic moments of the adatoms and dimers. Contrary to previous work, our results show that adatoms bind weakly to graphene with binding energies ranging from 0.2 to 1.4 eV depending on the adsorption site and species. For both homonuclear and heteronuclear dimers the binding energies per atom are lower than the respective adatom cases, ranging from 0.1 to 0.5 eV per metal atom. The most strongly bound configurations for all the dimers studied are those with the dimer axis (nearly) perpendicular to the graphene plane and bound at the hole site. These configurations, which, to our knowledge, have not been considered in previous work, also turn out to have the largest enhancement of the magnetic moment at least for the atom farther from the graphene. The binding energies of these most strongly bound dimers are dependent on three factors, namely, the interconfigurational energy change in the dimer atom farther from graphene upon desorption, the charge transfer from the dimer to the graphene, and the adsorption site favored by the atom closer to the graphene sheet. The first factor is dominant for all the dimers studied here except for CoPt and NiPt. The relatively high electronegativity of Pt affects the character of the charge transfer from the dimer to graphene. In most of the dimers we investigated, charge is transferred almost exclusively from the dimer atom closer to the graphene except for heteronuclear dimers with Pt where charge is also transferred between the two dimer atoms upon adsorption. Thus, our calculations of the electronic structure allow us to understand the trends in binding energy and the magnetic moment in these dimers.

Small Fe, Co and Ni clusters are known to exhibit high magnetic moments and are therefore investigated rather intensely for the purpose of developing novel magnetic materials with high magnetisation densities. The reduced bond density of these clusters compared to their respective bulk states, results in them being particularly sensitive to their environment. The choice of a substrate for these clusters is therefore of particular importance. Graphene has been shown to exhibit many novel phenomena and it is therefore interesting to investigate the suitability of using graphene as a support material for these metal clusters and if this would allow for an integration of technologies. In this paper, we report the results of plane–wave density functional theory (DFT) calculations of Fe, Co and Ni adatoms, and homonuclear and heteronuclear dimers, including mixing with Pt, adsorbed on graphene. We investigated the adsorption site structure, and stability, the projected density of states and electron populations, and magnetic moments. Calculations were performed using the Perdew–Burke–Ernzerhof (PBE) functional for the wavefunction with energy cutoffs of 40Ry and 480Ry for the wavefunction and density respectively. Brillouin zone sampling was performed with a Monkhorst–Pack grid of (8 ? 8 ? 1). We find that the adatoms bind weakly to graphene and that the magnetic moment of the most stable adatom configuration (the hole site configuration) is reduced by 2 ?B compared to the free adatom, and that the most stable dimer configuration is one where the dimer bond axis is oriented perpendicular to the graphene plane. The stability of the adatoms and dimers on graphene can be explained by considering the electronic interconfigurational energy change that accompanies the desorption of these clusters as well as the amount of charge transferred from cluster to graphene. Therefore, the accuracy of these calculations will depend rather strongly on how adequately the 3d–4s exchange correlation inter–action is treated.